The mechanical and chemical properties of iron-containing metals, alloys and steels depend upon the internal and external crystalline structure of the material, which may be altered by heat treatments such as annealing, normalizing, quenching and tempering. Heat treatments, particularly quenching and tempering, are commonly used to manipulate the internal microstructure of steel to obtain improved mechanical properties from a given steel composition. Quenching involves heating the steel above a critical temperature that is sufficiently high to form austenite (which differs depending upon the elemental composition of the steel), and then hardening the steel by quenching, in a medium such as oil, water, other liquids or blasts of a gas such as air, to cool the steel rapidly and thereby induce the formation of martensite or bainite, to provide a hard microstructure. The microstructure of heat-treated hardened steels gives the material a high strength but a low ductility. To increase the fracture toughness of the material, the as-quenched steel may be subjected to a second heat treatment called tempering. In tempering, the material is reheated to a temperature below the critical temperature for quenching, and maintained at the tempering temperature for a period of time to obtain the desired characteristics of strength and ductility. Some steels may not be amenable to these transformations of microstructure, and are not hardenable with heat treatment, such as very-low-carbon-containing and other ferritic and austenitic steels for which the critical quenching temperature for formation of austenite is below room temperature. The nature of steels that are hardenable with heat treatment is well known in the art, and the susceptibility of materials to hardening with heat treatment is readily determined empirically. In one aspect, the present invention relates to the treatment of any iron-based alloys including irons and steels that provide desired mechanical properties when quenched and tempered.
During heat treatment various oxides of iron may be formed on the surface of iron or steel, depending upon the temperature, length of heating, rate of cooling, and availability of oxygen during these various phases of potential oxide formation. In order of oxidation state of the iron, these oxides are ferrous oxide (FeO), called wustite or wuestite, magnetite (Fe.sub.3 O.sub.4) and ferric oxide, called hematite (Fe.sub.2 O.sub.3). The arrangement of these oxides on a surface may be complex, and may change with heat treatment, just as the internal microstructure varies with different heat treatments. The formation of the oxides may initially be determined by surface reaction conditions. Thicker oxide layers may evolve under heating through a process of oxygen diffusion within the oxide lattice. The structure of the oxide layers will affect the rate and extent of any such diffusion, and may be significantly affected by the presence of alloying elements such as chromium, aluminum or silicon. At high temperatures, for example above approximately 560.degree. C. for some steels, wustite will typically predominate in an oxide scale. At lower temperatures, wustite may decompose into iron or magnetite, and magnetite may be transformed into hematite.
In many circumstances, an external iron oxide layer (scale) is an undesirable byproduct of heat-treating steels or irons in an oxidizing environment, particularly the penetrating oxidation of plain-carbon or low-alloy steels. In some applications, however, a protective iron oxide coating may be desirable. For example, U.S. Pat. No. 4,035,200 issued to Valentijn on Jul. 12, 1977, discloses an innovation in the field of `blackening` processes that are used to produce a protective, dark oxide layer consisting predominantly of Fe.sub.3 O.sub.4 (magnetite) on an iron surface in a single step treatment in the range of 500.degree. C.-650.degree. C. The innovation disclosed therein apparently relates to the use of a particular oxidizing atmosphere comprised of combustion gasses flowing over the workpiece. Another process for obtaining a relatively homogeneous oxide layer, in this case FeO (wustite) is disclosed in International Patent Publication WO 99/10556, dated Mar. 4, 1999. U.S. Pat. No. 3,940,294, issued to Sergeant, Feb. 24, 1976, suggests that wustite may be a preferred oxide scale because it is relatively easy to remove, and that patent discloses a method of suppressing the transformation of a wustite scale to magnetite which may otherwise occur in making hot rolled steel stock. Academically, it has been suggested, however, that a thin, homogeneous wustite scale may be relatively resistant to fatigue microcrack propagation (C. V. Cooper and M. E. Fine, "Fatigue Microcrack Initiation in Polycrystalline Alpha-Iron with Polished and Oxidized Surfaces", Metallurgical Transactions A, vol. 16A, pp 641-649, 1985). Cooper & Fine disclose a process that produced 0.1-0.35 .mu.m thick FeO on polycrystalline pure iron using a controlled ratio of CO:CO.sub.2 flowing over the steel specimens kept at a temperature of 627.degree. C. This oxide layer was used to investigate the role of wustite (FeO) in the initiation of fatigue cracks in iron. An decrease in the fatigue life was noted as a result of the wustite layer.
Although the methods and products of the present invention are not limited to any particular practical application, one area of potential application is in the field of steel vessels for pressurized gases. For example, the natural gas vehicle (NGV) industry makes use of pressure vessels (cylinders) for the on-board storage of compressed natural gas (CNG) fuel. Steel cylinders are also used for the ground storage of CNG at fuelling stations. The interior surface of those steel cylinders and liners may be exposed to contaminants present in the natural gas, such as moisture (H.sub.2 O), carbon dioxide (CO.sub.2) and hydrogen sulphide (H.sub.2 S). Moreover, dynamic stresses are generated on the cylinder wall due to the pressure cycling of the vessel. Fuelling generates a high pressure, whereas, fuel consumption drops the pressure to a low value. This combination of the dynamic stresses and the corrosive contaminants causes corrosion fatigue to occur in NGV service, thus limiting the life of the steel cylinders and the steel-liners. For a given wall thickness, it may be possible to extend the life of the cylinder if the interior of the cylinder is protected from corrosion fatigue. Alternatively, a protective coating may facilitate the use of cylinders having thinner walls, permitting the use of less massive vessels to enhance vehicle fuel efficiency. Applying conventional protective coatings on the interior of a cylinder may, however, be somewhat difficult and costly.
Low alloy steels are generally used for the fabrication of NGV cylinders. A modified form of AISI/SAE 4130 (American Iron & Steel Institute/Society of Automotive Engineers) steel is often employed. Although the composition of the low alloy steel may vary, the composition shown in Table 1 is typical.
TABLE 1 Element Weight % C 0.35 Cr 0.80 Ni 0.30 Mo 0.15 Mn 0.50 Si 0.35 S 0.02
The fabrication process used for obtaining the desired shape of the vessel may vary. However, the shaping of the vessel is generally completed before the vessel is subjected to heat treatment. Prior to heat treatment, the shaped cylinder usually has a single narrow opening which is threaded for making the appropriate pressure connection in service. Alternatively, two narrow threaded openings may be provided, one located at each end of the cylinder.
Shaped steel pressure vessels are generally quenched by being heated to an austenitizing temperature, typically around 860.degree. C. for about 1-2 hours, and then rapidly cooled through immersion of the hot vessel into a liquid bath or a liquid spray. The quenching liquid may be water or an appropriate mixture of other chemicals. This process of quenching produces a hard microstructure (either martensite or bainite). This hard microstructure gives the cylinder material a high strength but a low ductility. In order to increase the fracture toughness of the material (to enhance safety of the pressure vessel in service) the as-quenched cylinder is subjected to a tempering treatment. In tempering, the cylinder is heated to a predetermined fixed temperature that is lower than the austenitizing temperature, which may lie between 500.degree. C.-650.degree. C. At this temperature the vessel may be maintained for 1-2 hours, before being cooled down to room temperature. The choice of a specific combination of the tempering temperature and time may be dictated by the hardness (strength) and ductility (or toughness) requirements for the final product. Normally, the cylinder interior remains untouched following the fabrication heat treatment. However, the exterior surface of the cylinder is cleaned (sand or grit blasting is commonly employed) and a coating is applied to the cylinder exterior in order to prevent atmospheric corrosion. There is however a need for processes that may be used to provide a protective coating on the interior of such cylinders.